Field guide
Superheat vs Subcooling: When to Use Each
Two readings, two different questions. Using the wrong one is how correctly charged systems end up overcharged.
Superheat is measured on the low side and describes how much heat the refrigerant picked up past the point where it finished boiling. Subcooling is measured on the high side and describes how much heat it gave up past the point where it finished condensing.
Which one applies to your system
| Metering device | Primary charging reading | What the other reading tells you |
|---|---|---|
| Fixed orifice / piston | Superheat | Subcooling indicates condenser and liquid-line condition |
| TXV / EEV | Subcooling | Superheat indicates whether the valve is controlling correctly |
Conditions that must be true first
- Airflow verified — measured static pressure and known blower CFM, not a visual check.
- Clean filter and clean coils, indoor and outdoor.
- System run long enough to stabilise, with indoor conditions recorded.
- Correct target values pulled from the manufacturer's charging chart for the measured conditions.
Skip any of these and the number you read is describing the conditions, not the charge.